Physiological strain due to load carrying.

In an experimental study of load carrying the effects of mass (0, 5.4, 10.4 kg) and the type of support (on the shoulder or on waist) on parameters of physiological strain were quantified to determine the factor(s) which limit carrying time. Four categories of strain were investigated: metabolic (in terms of oxygen uptake), cardiovascular (in terms of heart rate), muscular (in terms of EMG activity) and skin pressure under the shoulder straps. Four young male subjects were tested on a treadmill using different combinations of load and speed. While standing, oxygen uptake was not influenced by the type or mass of the backpack, and averaged 10% maximal oxygen uptake. The heart rate increased significantly by 9 beats per min while standing wearing a backpack, independent of type of support or mass of backpack. While walking both the heart rate and the oxygen uptake were significantly influenced by the mass carried, but both types of strain remained below the tolerance limits for prolonged wear. Standing supporting a load did not significantly increase the root mean square value of the EMG signal of the trapezius pars descendens muscle. While walking, load carrying significantly increased the root mean square value, and, converted to force, the largest increase amounted to 2.7% of the maximal force for a load of 10.4 kg suspended from the shoulders. This was below levels of force producing fatigue, which was also indicated by an absence of changes in the median power frequency of the EMG signal.(ABSTRACT TRUNCATED AT 250 WORDS)